Many different applications operate using optical techniques. Optical techniques allow many different wavelengths to be multiplexed on the same fiber. Each wavelength can represent a separate data stream. In some applications, it may be desirable to obtain parts of said data stream from the waveguide without obtaining other parts of the data stream from the waveguide. It may also be desirable to add new multiplexed data to the optical stream already present on the waveguide.
An add/drop multiplexer is often used to carry out the addition or removal of channels from an optical signal in a waveguide such as a fiber. Different kinds of add/drop multiplexers are known.